Hydraulic systems, in which power is transferred from one place to another, are used extensively in industrial equipment, farm equipment, transportation equipment, and the like. Illustrative of such equipment are lifters, jacks, elevators, mills, presses, and braking and power steering systems for vehicles.
High pressures and temperatures, which are frequently present in hydraulic systems, place high demands on the thermal and oxidative stability of the fluid used as the hydraulic medium. In addition, the lubricity of the hydraulic fluid is especially important with hydraulic systems wherein a pump is used to pressurize or move the hydraulic fluid from one place to another.
Polydiorganosiloxanes have been recognized as having exceptional thermal and oxidative stability, compatibility with seal materials, and high viscosity indices, said properties making them potentially useful as hydraulic fluids.
Unfortunately, the generally low surface tension of polydiorganosiloxanes tends to cause them to have marginal lubricity on metals. As a result, additives improving the lubricity of polydiorganosiloxanes have been sought.
Groenhof et al., in U.S. Pat. No. 3,759,827 disclose the use of a chlorendate diester to improve the lubricity of a polydiorganosiloxane fluid.
Page et al., U.K. Pat. No. 1,535,265, disclose improved silicone hydraulic fluids which comprise a siloxane fluid, a chlorendate diester, and a lubricant additive compound selected from dithiocarbamates and phosphorodithioates of antimony and lead. The stability of these additives to settling at room temperature and below room temperature, however, is limited.
Holbrook et. al., in U.S. Pat. No. 4,137,189, disclose improved silicone hydraulic fluids which comprise a non-linear siloxane fluid, a chlorendate diester and a lubricant additive compound selected from dithiocarbamates and phosphorodithioates of antimony and lead. The compositions of Holbrook et al. have enhanced stability to settling, as evidenced by improved cloud point temperatures. However, a non-linear siloxane of the type used by Holbrook et al. is more expensive to manufacture than a linear polymer. In addition, the concentration of additives it is possible to incorporate into the compositions of Holbrook et al., and still retain a non-settling hydraulic fluid, is still somewhat limited.
Martin, in U.S. Pat. No. 4,155,864 discloses the incorporation of small amounts of polydimethylsiloxane gum into silicone dielectric fluids. Said incorporation can also be beneficial in other silicone compositions, such as heat transfer fluids, hydraulic fluids and the like.
Although the silicone hydraulic fluid compositions of the art discussed above have been widely accepted, there still exists a need for a silicone hydraulic fluid composition which has better stability to settling at room temperature and at lower temperatures. There also exists a need for an additive concentrate composition which can be used to rejuvenate spent silicone hydraulic fluid compositions.
In large hydraulic equipment installations, common accumulators and reservoirs for hydraulic fluid are often used. To compensate for leakage losses, it is convenient to add makeup fluid to the reservoir as it is needed. In addition, since the lubricant additives can become depleted through use, it would be highly desirable to have a composition more concentrated than the fluid with respect to the lubricant additives. Such a concentrate could be used both to replenish the reservoir, and at the same time, to raise the total concentration of lubricant additives to the desired level. So far as is known, such a concentrate is not currently available because of the settling problem discussed above.